The present invention relates to a screw centrifuge for wet mechanical separation of solids according to their density.
This is generally implemented by a rotating, cylindrical drum with two conical drums connected to it and openings for discharge of the separated material as settleable solids and floating solids and of the separating liquid. A rotating shaft with openings for infeed of the solids to be separated is provided, where the shaft has two screw flights working in opposite directions. A baffle disc is arranged on the shaft in axial direction between the infeed opening for the substances to be separated and the discharge opening for the settleable solids. The outer flight of the screw for the settleable solids surrounds the inner flight of the screw for the floating solids, and the flights work in opposite directions.
Screw centrifuges of this kind are known from EP 0 553 793 B1. Machines of this type have a limited throughput for the light or floating fraction. If the established throughput is increased, the machine becomes blocked with the product. Other similar machines are known from DE 195 16 636 A1 or EP 1485 205 B1. These types of machine are used for three-phase separation of substance mixtures, e.g. different plastic fractions and a separating liquid. By choosing suitable densities of separating liquid, it is also possible to separate into several different fractions in multi-stage processes. In addition, DE 3134935 A1 describes a decanter centrifuge for separating a solids phase and two liquid phases. In this case, it is not possible to separate a light or floating fraction. The decanter centrifuge in DE 2612696 also cannot separate floating solids. It separates floating and heavy solids that are deposited on or carried by gravity onto the inner circumference of the centrifugal drum. U.S. Pat. No. 2,528,974 describes a centrifugal separator for separating solids and liquids, but three-phase separation is not possible here either. The spiral separator in U.S. Pat. No. 4,781,822 separates two kinds of particles according to density or size.